1. Field of the Invention
The present invention relates to a quasi resonant type switching power supplying unit equipped with an overcurrent protection circuit.
2. Description of the Related Art
As shown in FIG. 7, a conventional quasi resonant type switching power supplying unit 100 includes an integrated circuit Q, a MOSFET 1, and a MOSFET 2 that are housed in a package P. The quasi resonant type switching power supplying unit 100 has a Vcc terminal T1, a TRG terminal T2, an FB terminal T3, a SOURCE terminal T4, a DRAIN terminal T5, and a VREF terminal T6. A source electrode of the MOSFET 1 is connected to the SOURCE terminal T4. A drain electrode of the MOSFET 1 is connected to the DRAIN terminal T5. The drain electrode of the MOSFET 1 is also connected to a drain electrode of the MOSFET 2.
In the integrated circuit Q, an oscillation circuit 3 generates a saw-like oscillation signal. When a charge control signal is input from a fixing circuit 4, a MOSFET 5 switches to an ON state and charges a capacitor C in the oscillation circuit 3. With this arrangement, the charge time of the capacitor C can be shortened. An oscillation edge circuit 8 converts the saw-like signal into a pulse signal and outputs the converted pulse signal to a latch circuit 9.
The fixing circuit 4 produces the charge control signal as a result of a logic operation applied to a trigger signal TRG and a frequency restriction signal. The trigger signal TRG is input from the trigger terminal T2 in response to a signal generated from an auxiliary winding of an external transducer. An edge detecting circuit 6 detects the trigger signal TRG. A signal of the oscillation circuit 3 is fed back to the fixing circuit 4. The fixing circuit 4 generates the frequency restriction signal when a predetermined time has passed after switching of the oscillation signal from a high level (H) to a low level (L), or vice versa.
A reference voltage generating circuit 10 receives a power voltage Vcc from the Vcc terminal T1 and generates a reference voltage Vref. An external capacitor is connected to the quasi resonant type switching power supplying unit 100 via the VREF terminal T6. The power voltage Vcc is supplied to the external capacitor, and a reference voltage Vref is obtained from the charge voltage of the external capacitor.
A low-voltage detecting circuit 11 compares the power voltage Vcc with a threshold voltage UVL. When the power voltage Vcc is less than or equal to the threshold voltage UVL, the low-voltage detecting circuit 11 outputs a stop signal to the oscillation circuit 3 and to a driver circuit 18. When the power voltage Vcc is greater than or equal to a threshold voltage UVH, the low-voltage detecting circuit 11 outputs a stop reset signal to the oscillation circuit 3 and to the driver circuit 18. The oscillation circuit 3 stops generating the oscillation signal upon receiving a stop signal and restarts generating the oscillation signal upon receiving a stop reset signal. In this manner, the low-voltage detecting circuit 11 has two threshold voltages UVL and UVH to provide a hysteresis (i.e., a voltage difference) at both the start and the end of the generation of the oscillation signal.
A high-voltage detecting circuit 12 compares the power voltage Vcc with a threshold voltage OVP. When the power voltage Vcc is greater than the threshold voltage OVP, the high-voltage detecting circuit 12 outputs a stop signal to a latch circuit 9. The latch circuit 9 latches a signal produced from the oscillation edge circuit 8 in response to the stop signal. A state-of-overheat detecting circuit 13 detects the temperature of the quasi resonant type switching power supplying unit 100 and outputs a stop signal to the latch circuit 9 when the temperature exceeds a predetermined reference temperature. The latch circuit 9 latches a signal generated from the oscillation edge circuit 8 in response to the stop signal received from the state-of-overheat detecting circuit 13.
An oscillation level comparison circuit 15 receives a detection voltage VCOM (i.e., a voltage containing a load voltage input from the FB terminal T3 and a voltage input from the sensing MOSFET 2), and compares the detection voltage VCOM with the reference voltage Vref supplied from the reference voltage generating circuit 10. When the detection voltage VCOM is less than or equal to the reference voltage Vref, the oscillation level comparison circuit 15 outputs a low-level reset signal. When the detection voltage VCOM is greater than the reference voltage Vref, the oscillation level comparison circuit 15 outputs a high-level reset signal.
A pulse width modulation circuit 16 is a RS flip-flop circuit having a set terminal S, a reset terminal R, and an output terminal Q. A pulse signal having passed the latch circuit 9 is input via a buffer 17 to the set terminal S of the pulse width modulation circuit 16. An output signal of the oscillation level comparison circuit 15 is fed to the reset terminal R of the pulse width modulation circuit 16. A signal produced from the output terminal Q of the pulse width modulation circuit 16 is supplied via the driver circuit 18 to a gate electrode of the MOSFET 1 and to a gate electrode of the MOSFET 2.
The conventional quasi resonant type switching power supplying unit 100 uses the power voltage Vcc to charge the capacitor and obtains the reference voltage Vref from the charge voltage. In such a case, a required capacitance is in the range of several hundreds nF to several μF. In general, such a large-sized capacitor cannot be incorporated in the quasi resonant type switching power supplying unit 100. Accordingly, it is necessary to provide the VREF terminal T6 to which the external capacitor is connected.
In general, providing the VREF terminal dedicated to the external capacitor increases a package size of the quasi resonant type switching power supplying unit 100. Furthermore, when the quasi resonant type switching power supplying unit 100 is mounted on a circuit, a process for connecting a capacitor to the circuit is necessary and accordingly the manufacturing process becomes complicated and the manufacturing cost increases.
Furthermore, if a load is short-circuited in the conventional quasi resonant type switching power supplying unit 100, the capacitor connected to the VREF terminal T6 is continuously charged and the reference voltage Vref is maintained at a high voltage value. Accordingly, the oscillation level comparison circuit 15 cannot detect a problem occurring in the circuit. Thus, power supply to the load cannot be stopped and the quasi resonant type switching power supplying unit 100 is brought into an excessive load condition.